This research is designed to investigate nutritionally-induced alterations in hormones and their impact on the-nutritional requirements of humans. It is specifically designed to investigate the effects of energy_balance and imbalance and diet composition on thyroid hormone and catecholamine metabolism, and how thermogenesis (energy expenditure) is regulated. A new hypothesis is proposed to explain how thyroid hormone metabolism and sympathetic nervous system activity are regulated by energy balance and caloric flux. Nutritionally-induced alterations in hormones and sympathetic nervous system activity will be correlated with changes in thermogenesis to assess their role(s) in the adaptation of the thermogenic response to altered nutritional states and whether obesity results from a defect(s) in these adaptive responses. In vivo and in vitro estimates of muscle Na+, K+-ATPase activity will be correlated with nutritionally-induced alterations in thermogenesis, plasma thyroid hormone and norepinephrine metabolism.